I have been playing Age of Empires - Conquerors for more than 6 years. We used to compete each other using LAN. What we generally experience is, with population limit of 100 or more, the Korean war wagon seems to be indestructible because of its large hit point and good attack. What would be a good strategy to defeat it?

Pikemen are especially effective if they manage to get close. A practical approach though, is to use cavalry - especially light cavalry which are somewhat less susceptible to missile damage. Cavalry can close in fast and quite effective in groups.

Massed war wagon are pretty hard to beat and will take down much of your pikemen or cavalry before they get close. One alternative approach that I've seen used (but never used myself) is to use a small group of onagers. The onagers are kept behind other units for protection so they don't get attacked right away, and they all attack at once once they get in range. This only works if they are fully upgraded, and even then this is basically a sacrificial strategy, you you will end up losing your onagers to the war wagons that survive the initial attack. But it does destroy a number of war wagons and weaken the others, and give a better chance for other units to close in.

Use mass Skirmishers, preferably with Onagers behind them. If required, some Light Cavalry can also be expended to soften them up. I feel this is the best approach keeping in mind the costs involved and the fact that any civilization can implement this counter..

Keep in mind that you will lose quite a few units, but I have used this several times, and think it works best. Skirmishers are easily replaceable. And since War Wagons are basically (very) beefed up Cavalry Archers, you get a neat bonus while attacking them

Warwagons are extremely powerful. As a matter of fact, the Korean civilization used to get banned in LAN plays/leagues when Age of Empires was a hit back in my days.

I've countered them with Camels; its the best way, and don't forget that Korean units can be converted/do not die, so conversion along with camels is best way to get them.

But strategically the best way I defeated them was using Chinese/Mayans/Britains. Not due to archers or damage, but a fast game where I attack the Koreans before they hit the Castle Age, and keep them under pressure all the time by capturing much of their resource points.